1. Field of the Invention
The present invention relates to an R-T-B based permanent magnet whose main components are a rare earth element (R), at least one or more kinds of iron element essentially including Fe or Fe and Co (T), and boron (B).
2. Description of the Related Art
R-T-B based permanent magnets have excellent magnetic properties and are used for home electric appliances, various kinds of motors such as voice coil motors (VCM) of hard disk drive and motors mounted on hybrid cars, and the like. When the R-T-B based permanent magnet is used for the motor or so, it is required to have a high coercivity for responding to a use environment of high temperature.
When the R-T-B based permanent magnet is assembled in products of motors or so, this handling is difficult in a magnetized state, and thus a magnetization treatment is usually carried out after the R-T-B based permanent magnet is assembled in the products. In a state where the R-T-B based permanent magnet is assembled in the product, it is impossible to apply a large external magnetic field compared to when manufacturing the R-T-B based permanent magnet, and magnetization properties thus need to be heightened as well as coercivity.
Miniaturization of main-phase crystal grains of the R-T-B based permanent magnet is well known as a method for improving coercivity of the R-T-B based permanent magnet. This mechanism is not completely clear, but it is conceivable that the miniaturization of the main-phase crystal grains contributes to reduction of defect around crystal grain boundaries to be generation source of reverse magnetic domains. In case of miniaturization of the main-phase crystal grains, however, there is a problem of deterioration of magnetization properties.
Patent Document 1 discloses a technique for obtaining favorable coercivity and magnetization properties in a manner that a median of grain sizes of crystal grains on a cross section vertical to a c axis of an NdFeB sintered magnet is 4.5 μm or less, and that an area ratio of crystal grains whose grain sizes are 1.8 μm or less is 5% or less on the cross section. There is, however, a problem that when the main-phase crystal grains are further miniaturized, an area ratio of the crystal grains whose grain sizes are 1.8 μm or less cannot be 5% or less, and favorable magnetization properties cannot be obtained.    Patent Document 1: WO 2014/034650    Non-patent Document 1: Soshin, Chikazumi (1984) “Physics of Ferromagnetism, Vol. II”: SHOKABO Co., Ltd.